This invention pertains to fluorinated metal-organic frameworks having internal channels and cavities in a variety of configurations that are capable of adsorbing and desorbing gases and molecules. This invention also pertains to gas storage in fluorinated metal organic frameworks, and more particularly to hydrogen storage.
Crystalline porous materials, either with an inorganic or a metal-organic framework (“MOF”), can be used in a range of applications. These include size- and shape-selective catalysis, separations, gas storage, ion-exchange, sensors, and optoelectronics. In particular, stable MOFs with permanent highly-porous channels or cavities have been explored as effective, economic, and safe on-board vehicular gas (hydrogen or methane) storage materials for fuel-cell-driven automobiles. Extensive efforts have been devoted to the rational design and construction of new MOFs with zeolite-like, well-defined, stable and extra large micro or meso pore size channels exhibiting higher or selective gas affinity properties. Pioneered by Yaghi et al., a vast number of organic ligands with a variety of donor groups and over 40 metal cations have been explored in MOF construction (Yaghi, et al. 1995). A few reports on MOFs utilizing non-fluorinated metal triazolates have appeared recently. (Yang, et al. 2004; Zhang, et al. 2004, 2005; Ouellette 2006).
High volumetric capacity is a very significant property for gas storage applications. The U.S. Department of Energy (“DOE”) has established a multi-stage target for hydrogen storage capacity in materials, including those materials intended for fuel cell applications. The DOE's 2010 targets for a hydrogen-storage system are an energy density of 7.2 MJ/kg and 5.4 MJ/L. Energy density refers to the amount of usable energy that can be derived from the fuel system. The figures include the weight and size of the container and other fuel-delivery components not just the fuel. The 2010 values work out to be 6 wt % of hydrogen and 45 kg of hydrogen per cubic meter. For 2015, the DOE is calling for fuel systems with 9 wt % of hydrogen and 81 kg of hydrogen per cubic meter, which is greater than the density of liquid hydrogen (approximately 70 kg/m3 at 20 K and 1 atm). Particularly for H2 storage in automobiles, the volumetric capacity is arguably more important than the gravimetric capacity because smaller heavy cylinders are easier to accommodate in vehicles than larger cylinders even if the latter were lighter than the former. Due to their high porosity, the best metal-organic frameworks known to date have very low densities (e.g., 0.43, 0.51, and 0.62) (Yaghi, et al. 2006; Long, et al. 2006). Therefore, their volumetric densities are always lower than the gravimetric densities.
In attempts to meet the DOE targets, nanostructured carbon materials (e.g. carbon nanotubes, graphite nanofibers, activated carbon, and graphite) and porous metal-organic frameworks have become of interest to researchers as potential hydrogen adsorbents. However, it has been shown that nanostructured carbons have slow uptake, exhibit irreversible adsorption, and contain reduced transition metals as impurities. Meanwhile, the known MOFs have low volumetric H2 uptake due to their low densities and weak affinity to hydrogen molecules. In addition, the porous nature and high surface areas of metal-organic frameworks give rise to rather weak H2 adsorption energies (˜5 kJ/mol). This is why cryogenic temperatures are usually required to observe significant H2 uptake.
What is needed, therefore, is a MOF that is stable and capable of adsorbing a high volume of gases at higher temperature.